Simultaneous Interfacial Defect Passivation and Bottom-Up Excess PbI2 Management via Rubidium Chloride in Highly Efficient Perovskite Solar Cells with Suppressed Hysteresis.

In halide perovskite solar cells (PSCs), moderate lead iodide (PbI2) can enhance device efficiency by providing some passivation effects, but extremely active PbI2 leads to the current density-voltage hysteresis effect and device instability. In addition, defects distributed on the buried interface of tin oxide (SnO2)/perovskite will lead to the photogenerated carrier recombination. Here, rubidium chloride (RbCl) is introduced at the buried SnO2/perovskite interface, which not only acts as an interfacial passivator to interact with the uncoordinated tin ions (Sn4+) and fill the oxygen vacancy on the SnO2 surface but also converts PbI2 into an inactive (PbI2)2RbCl compound to stabilize the perovskite phase via a bottom-up evolution effect. These synergistic effects deliver a champion PCE of 22.13% with suppressed hysteresis for the W RbCl PSCs, in combination with enhanced environmental and thermal stability. This work demonstrates that the interfacial defect passivation and bottom-up excess PbI2 management using RbCl modifiers are promising strategies to address the outstanding challenges associated with PSCs.

Multifunctional Polymer Restraint of the Agglomeration of SnO2 Nanocrystals for Efficient and Stable Planar Perovskite Solar Cells.

The aggregation of SnO2 nanocrystals due to van der Waals interactions is not conducive to the realization of a compact and pinhole-free electron transport layer (ETL). Herein, we have utilized potassium alginate (PA) to self-assemble SnO2 nanocrystals, forming a PA-SnO2 ETL for perovskite solar cells (PSCs). Through density functional theory (DFT) calculations, PA can be effectively absorbed onto the surface of SnO2. This inhibits the agglomeration of SnO2 nanocrystals in solution, forming a smoother pinhole-free film. This also changes the surface contact potential (CPD) of the SnO2 film, which leads to a reduction in the energy barrier between the ETL and the perovskite layers, promotes effective charge transfer, and reduces trap density. Consequently, the power conversion efficiency (PCE) of PSCs with a PA-SnO2 ETL increased from 19.24% to 22.16%, and the short-circuit current (JSC) was enhanced from 23.52 to 25.21 mA cm-2. Furthermore, the PA-modified unpackaged device demonstrates better humidity stability compared to the original device.

Potassium Salt Coordination Induced Ion Migration Inhibition and Defect Passivation for High-Efficiency Perovskite Solar Cells.

The disordered distribution of trap states and ion migration limit the commercial application of perovskite solar cells (PSCs). Herein, we apply an oxamic acid potassium salt (OAPS) as a bifunctional additive of perovskite film. The Lewis base group C=O of OAPS can interact with the uncoordinated Pb2+ caused by the I site substitution by Pb and the dangling bonds of the perovskite, which is beneficial to reduce the nonradiative recombination loss. In addition, the countercation K+ of OAPS is confirmed to occupy the perovskite lattice interstitial sites and result in lattice expansion, inhibiting the formation of iodide Frenkel defects and I- ion migration. As a result, the synergistic effect achieves enhanced power conversion efficiency (PCE) from 19.98 to 23.02%, with a fill factor reaching up to 81.90% and suppressed current-voltage hysteresis. The device also presents improved stability, maintaining 93% of the initial PCE after 2000 h of storage.

Enhanced Hole Mobility and Decreased Ion Migration Originated from Interface Engineering for High Quality PSCs with Average FF beyond 80.

Perovskite solar cells (PSCs) have made significant progress in power conversion efficiency (PCE) by optimizing deposition method, composition, interface, etc. Although the two-step method demonstrates the advantage of being easy to operate, too much residual PbI2 not only forms defect centers, but affects the perovskite crystallization by arising more grain boundaries (GBs) due to the easy-to-crystallize nature of PbI2 . And GBs in polycrystalline perovskite usually provide main channel for ion migration, leading to accumulation of charges at the interface to form a barrier, thus reducing carrier mobility and resulting in degradation of perovskite devices. Here, an organic molecule N-(4-acetylphenyl)maleimide (N-APMI) is used to modify interface between perovskite and hole transport layer. X-ray photoelectron spectroscopy, scanning electron microscope, and nuclear magnetic resonance results show that ketone group (CO) in N-APMI forms a strong coordination with Pb2+ , which effectively reduces the residual amount of PbI2 nanoparticles on the perovskite surface, giving rise to improved crystallization of perovskite. Temperature-dependent current response demonstrates that ion migration is effectively suppressed, and hole mobility validly increases from 10.74 to 19.48 cm2 V-1 s-1 , leading to a champion fill factor (FF) of 82.5% (PCE 21.96%), and the maximum PCE of the device improves from 20.09% to 23.03%.

Enhanced Activation Energy Released by Coordination of Bifunctional Lewis Base d-Tryptophan for Highly Efficient and Stable Perovskite Solar Cells.

Perovskite defect passivation with molecule doping shows great potential in boosting the efficiency and stability of perovskite solar cells (PSCs). Herein, an efficient and low-cost bifunctional Lewis base additive d-tryptophan is introduced to control the crystallization and growth of perovskite grains and passivation defects. It is found that the additive doped in the solution precursors could retard crystal growth by increasing activation energy, resulting in improved crystallization of large grains with reduced grain boundaries, as well as inhibiting ion migration and PbI2 aggregation. As a result, the PSCs incorporated with d-tryptophan additives achieve an improved power conversion efficiency from 18.18 to 21.55%. Moreover, the d-tryptophan passivation agent improves the device stability, which retains 86.85% of its initial efficiency under ambient conditions at room temperature after 500 h. This work provides Lewis base small-molecule d-tryptophan for efficient defect passivation of the grain boundaries toward efficient and stable PSCs.

Strong Electron Acceptor of a Fluorine-Containing Group Leads to High Performance of Perovskite Solar Cells.

Organic-inorganic hybrid perovskites have become one of the most promising thin-film solar cell materials owing to their remarkable photovoltaic properties. However, nonradiative recombination of carriers usually leads to inferior performance of perovskite (PVK) devices. Interface modification is one of the effective ways to improve separation of charges for perovskite solar cells (PSCs). Here, a small organic molecule of tetrafluorophthalonitrile (TFPN) is used to enhance the extraction and transportation of carriers at the PVK/hole transport layer (HTL) interface. The electron-rich C-F group effectively reduces the trap state density in the perovskite through chemical combination with the empty orbital of Pb2+ or other electron traps on the PVK surface, resulting in enhanced interface contact between the PVK and HTL. Meanwhile, the C≡N group in TFPN also inactivates the defects caused by Pb2+. The Fermi level of the perovskite shifts by 0.15 eV to its valence band due to the strong electron acceptor nature of the F atom, indicating that positive dipoles and p-type doping emerge, which validly suppress the recombination of carriers for the PVK film. Therefore, the optimized PSC shows the highest power conversion efficiency (PCE) of 22.82% compared to 19.40% for the control one. The champion FF reaches up to 81.2% (PCE 21.44%) due to the effectively enhanced carrier separation. In addition, the unencapsulated device shows enhanced stability under air conditions.

Field emission from few-layer graphene nanosheets produced by liquid phase exfoliation of graphite.

Graphene nanosheets have been synthesized from commercial expandable graphite by heating in a microwave oven and dispersing in ethanol by ultrasonication. Scanning and transmission electron microscopy and electron energy-loss spectroscopy and atomic force microscope showed that the nanosheets were about 2 nm in thickness and 10 microm in diameter. The field emission of the graphene sheets has been investigated. An emission current density of 1 mA/cm2 has been achieved at an electric field of 3.7 V/microm with a turn-on field of 1.7 V/microm at 0.01 mA/cm2. The annealing of the samples at 400 degrees C in vacuum greatly improved the field emission performance.